DELIBERATE TRACER STUDIES

Objective:
Deploy deliberately injected tracers to study dispersion and air-water gas
exchange in inland waters (estuaries and lakes).
Description:
Gas exchange and dispersion are two major mechanisms to decrease concentrations
of (semi)-volatile pollutants such as PCB's in inland waters. Study of
these processes in the field has been hampered by lack of suitable
methods. This new method uniquely addresses these processes.
Method:
The tracer sulfur
hexafluoride (SF6) is
injected into a body of water and the tracer patch is followed with
time. Since SF6 can be measured down to one part per
trillion, it is possible to study these processes on
large scales (103 km2) and over long periods of
time (months). Accomplishment:
Dispersion and gas transfer in
estuaries are of particular interest as they often transect large
industrial areas, and the mechanism of mixing is a unique combination of
tidal mixing, stream flow, and wind generated surface turbulence. A large
scale tracer experiment in the upper Hudson Estuary was led by Dr. Clark
of LDEO in which the processes were investigated. Gas exchange wind
speed relationships (Figure 1) suggest that
tidal mixing has
little influence on gas transfer in the upper Hudson. Lateral dispersion
determined from the change in the shape of surface concentration profile
with time show distinctly lower mixing rates in the first few days when the
patch is relatively small (Figure 2)
Key reference:
Clark, J.F., P. Schlosser, R. Weppernig, M. Stute, R. Wanninkhof,
and D. Ho, Relationship between gas transfer velocities and wind
speeds in the tidal Hudson river determined by the dual tracer technique,
in Air-Water Gas Transfer: Selected papers from the Third International
Symposium on air-water gas transfer, edited by B. Jaehne, and E. Monahan,
pp. 785-800, Aeow Verlag, Hanau, Germany, 1995.